How do you know if a horse will be a "trier" pre "off".

Give up trying to work it out. The majority of horses run on their merits and the buzzers/non-buzzers will even themselves out just like luck/bad luck does. People always get their pants in a twist on here when drifters run poorly but huge drifters win races day in day out, it's just the mugs choose to ignore it when this happens.

Put yourselves in the shoes of the owners. How much do you want to win? Is your aim to defray training fees? Do you want to win a race w/ good prize money or will you feel you have to go for a betting touch in any sort of race?

If you want to win a lot, you might want the price to drift before you put your bets on. Alternatively, you might want to take advantage of a good morning price for a smaller outlay and smaller reward.

If you research the betting patterns of yards and even owners, you will begin a clear difference between certain 'gambling' yards and stables where little can be read into the price, whether drift or steam. You can see how well the horse did in the race by keeping records of the IR price.

Mostly, though, by the time this market move has been picked up by the broader market, any value in the price is gone.

You'll probably find that most are trying but it's the one's that aren't that you need to know. Unfortunately it's only with hindsight usually that you can spot the real non triers. The ones that drift drastically just before the off, miss the break, run wide on the bend then if there's the remotest chance of winning are quickly brought back inside to run up the back of horses in front. All accompanied by (apparently) furious riding by the jockey.
Stewards however seem to think this is quite normal.